1. Field of the Invention
The present invention relates generally to a high flash point composition and a safe method for treating fuel-burning boilers to control emissions and deposits.
More particularly, a high flash point composition and a safe method for treating fuel-burning boilers to control emissions, where the composition comprises a high flash point liquid including oil-soluble nano-particle size additive particles for fuel such as coal to reduce emissions and deposits.
2. Description of the Related Art
In the generation of electricity the often preferred fuel is natural gas. Natural gas is preferred because it contains very few if any contaminants. Contaminants often cause problems in the power generating industry. Some of these problems result from deposits that accumulate on heat transfer surfaces in boilers, the moving blades in gas turbines, or the moving pistons and valves in diesel engines. These deposits reduce the effectiveness of these units and thereby increase the need to combust additional fuel to compensate for the loss in effectiveness. Other problems encountered are the production of environmental contaminants in the exhaust of these units such as SOx, SxOy, NOx, NxOy, and heavy metals that are often discharged to the environment when adequate treatment is not possible.
In the absence of relatively scarce and valuable natural gas, electric utilities are often forced to rely on less environmentally friendly fuels. These fuels may be derivatives of petroleum taken from various stages of the refining process of this increasingly expensive commodity. When price per unit of derived energy is considered, the chosen fuel is frequently coal when it is available as a viable option. Of all the fuels coal has by far the greatest number and largest amounts of contaminants and thus has the greatest environmental impact when not properly and completely treated.
To solve various aspects of treating oil and coal in power generation the metal elements magnesium, calcium, iron, manganese, sodium, potassium, zinc, cerium, barium, silicon, aluminum, chromium, cobalt, nickel, and copper have been tested. Many of these materials have consequently been used as active metal additives for many and varied uses in fired equipment such as boilers, gas turbines, and diesel engines. For ease of handling, these elements are often applied as liquid formulations. For years, crude slurry formulations have reigned supreme in boiler treating. More recently, oil soluble formulations with nano-sized particles have become increasingly more common as their value has been seen in boiler treating. In many of these applications, the “liquid” additive is simply injected into the fuel being used at locations that are remote from the combustion equipment. In other applications it may be necessary that the “liquid” fuel additive is sited and injected near the combustion equipment. As a result it has become customary for minimum flash point specifications to be placed on these various additive formulations. Higher flash points are more desirable, but it has previously been thought that the additional cost or resulting degradation of product properties to achieve that goal is not warranted.
From this point on the flash point of a liquid fuel will be defined as lowest temperature at which it can form a mixture that is ignitable in air due to increased vapor pressure. Once the source of ignition is removed, the vapor of the liquid will stop burning. This is sometimes confused with autoignition temperature which is the lowest possible temperature at which the fuel liquid will spontaneously ignite under STP conditions without an external source to supply the activation energy needed to combust.
Even with the hazards associated with most of these “liquid” formulations, the problems the additives are known to solve have made the dangers acceptable under some conditions. Many of these materials containing the elements mentioned above find many uses in solving power plant problems. Each has its place in the pantheon of fuel treatment to provide the most effective combustion of the various fuels with minimal contribution to world-wide pollution.
Alkaline compounds (magnesium and calcium in particular) have been used since the early 20th Century for treating fuel combustion problems, specifically in steam boilers. The problems being treated were caused principally by iron, potassium, sodium, sulfur, and/or vanadium contaminants existing in the fuel, whether the fuel was coal or oil. The compounds used for this treatment have traditionally been calcium and magnesium oxide and/or hydroxide both in powder form and/or as oil dispersed slurries. These slurry materials often caused more difficulties than they solved due to product problems. Because of product settling, shipping containers needed to be mixed before use; storage tanks needed to be constantly circulated to avoid subsequent product settling; pumps or blowers used to feed the material often wore out from product-caused erosion. Large product excesses were specified and required to effect treatment because of large product particles. Even with all these inadequacies these products have been used for many years mainly because they were regarded as inexpensive.
Another use of calcium and particularly magnesium oxide is in preventing sulfur-caused problems in boilers. Sulfur contained in the typical fuels for this application will convert to sulfur dioxide during combustion. A portion of this sulfur dioxide continues to oxidize with additional oxygen to form sulfur trioxide. Upon cooling, sulfur trioxide combines with water vapor—also formed during combustion—to form corrosive sulfuric acid. While still in combustion equipment, this acid readily corrodes iron. After exhausting combustion equipment, this acid is a major source of acid rain. The chemical process to convert sulfur to sulfur trioxide from sulfur dioxide requires a catalyst for conversion—typically hot metal surfaces. Magnesium oxide has been shown to interfere with this catalyst action by passivating hot metal surfaces that normally act as these catalysts. As an added benefit, magnesium oxide also interacts with sulfur trioxide or sulfuric acid that may still form to neutralize them. Thus the use of magnesium compositions in combustion equipment reduces operator problems with corrosion and also helps the environment.
Calcium has been used chiefly to remove sulfur oxides from flue gases by the formation of thermally stable calcium sulfate. In this role the calcium solids have often been added just before the economizer. Often calcium has been used in the form of either ground limestone or lime, often associated with petroleum distillates to assist handling.
Iron compounds have also been used in boilers to reduce the amount of unburned carbon. Iron has a catalytic effect. Molecules of hydrocarbon and oxygen can combine more readily on surfaces of iron atoms to produce the desired carbon dioxide and water vapor of combustion. Freshly prepared, small particle iron—and iron oxides—have been found to be particularly effective for this catalyst effect. Iron for this purpose has typically been added on the coal in less effective forms. Oil-soluble iron formulations have been used in petroleum applications with positive effects. Other metallic elements have also been used to treat these various combustion and contaminant problems. For example, manganese, cerium, and barium have all been used as combustion improvers alone and in combination with each other and other metal elements. Zinc, silicon, aluminum, and copper have been used to modify and eliminate deposits.
In short, many different elements have been used quite effectively for many years to protect power generating equipment and to obtain maximum output of electricity from each unit of fuel.
In most cases the treating elements have been delivered in liquid form. This form may be as simple as a solid dispersed into an oil carrier for ease of handling or as complex as a true oil-soluble material to provide the smallest possible particle size (nanometer-sized), with the largest possible surface area to facilitate reactions in the combustion zone of a steam boiler, gas turbine, or diesel engine.
In nearly every application, it is imperative to obtain the best possible dispersion of the chosen element into the fuel. With liquid fuels, this has been as simple as injecting the additive into the fuel prior to combustion. With coal, the optimum treatment may be spraying onto the coal as it is moves on the converyor belt prior to passage through any pulverizers in the system or spraying into the boiler itself. In either case, because low flash liquids have been used, attention needs to be paid to the physical location of the additive storage container, additive pumps used to deliver the additive of choice, and the environment around the coal after treatment. Hazardous conditions may be caused by lower flash point additives.
Some common solvents used with fuel additives are listed in Table 1.
TABLE 1Common Solvent Flash Points for Fuel AdditivesSolventFlash PointAromatic65° C.#2 Diesel52° C.#4 Diesel54° C.#1 Fuel Oil38° C.-72° C.#2 Fuel Oil52° C.-96° C.#4 Fuel Oil 61° C.-116° C.
Fuel oils of higher number (#5 or #6) become too viscous to be considered as solvents for additives. This increased viscosity would simply exchange problems—low flash with difficulties in handling. The aromatic solvent shown would be a typical additive solvent that currently meets all flash point requirements for power plant applications. For solvents, these distinct physical properties typically exhibit the following trend: flash point<autoignition temperature.
The minimum 65° C. flash point is as much a requirement of the transportation industry as it is the power plant. To qualify for a combustible label instead of a flammable label, a product must have a minimum 65° C. flash point. Lower flash point materials make over-the-road tankers and trucks filled with drums of material more hazardous. The recognition of the increased hazards normally raises the cost to ship lower flash point products. Similarly, higher flash point materials will often carry significantly reduced shipping costs.
The primary additive form for many years to deliver the major treating element, magnesium, has been either as a fuel-oil-dispersed slurry of magnesium oxide or a water slurry of magnesium hydroxide. Similarly other metals have been delivered to the power plant as fuel-oil-dispersed slurries. These materials have all suffered from the major drawback of the slurry technology—relatively large sizes. A paradigm shift has been occurring during the past 10 or so years as the benefit of small—in fact nanosized particles—have become better known for fuel treatment. For example in treating boilers with a magnesium oxide slurry it has been established that for each part of contaminant present in an oil fuel, one part of magnesium from the slurry is required to achieve adequate treatment. Many of these slurry products will contain magnesium oxide particles in the one micron range (1μ, 1 μm, 1,000 nanometers). Conversely, using additives that contain nanosized particles—in every case a liquid, oil-soluble formulation—it has been demonstrated that only 0.2 parts of magnesium is required to adequately treat one part of contaminants. In some side-by-side tests, it has been found that even less might be possible.
The improvement in treating effectiveness is due to the increase of active surface atoms as the size of a particle decreases. For example, for a 10 nanometer particle of magnesia only 10% of the active atoms are located at the surface while for a 3 nanometer particle the number of active atoms located at the surface of the particle increases to 50%. See, e.g., “The Chemistry and Technology of Magnesia,” Mark Shand; Wiley Interscience: 2006. This relationship between size and active surface particles becomes very important, since many of the reactions that occur in boiler treating rely on surface activity of the particles. This theme is echoed in the EPA White Paper on Nanotechnology (EPA 100/B-07/001 February 2007). Other industry studies have also demonstrated that smaller particles allow for more effective treatment.
This has two immediate benefits to the power plant operator: 1) less material can be used to treat the same amount of fuel and 2) less ash waste is produced that requires disposal. Unreacted magnesium ends up in the ash pits in a boiler and is eventually hauled off to landfill or other disposal.
The chief benefit of slurry technology has always been assumed to be low cost. To maintain this low cost, only inexpensive solvent can be used. These tend to have the lower flash points as seen in Table 1. With the reduction in treating rates possible with the emerging oil soluble, nanoparticle technologies, however, many solvent choices are available.
A review of regulations for flash points and the hazards created by lower flash points demonstrates the value of higher flash point materials—especially during shipping over the road and in power plant usage. For example, the U.S. Department of Labor, Occupational Safety and Health Administration states under physical hazards that: “A chemical is a physical hazard if it: is likely to burn or support fire.” This same Hazard Communication Standard continues by stating that:                Flashpoint is the primary measure of a liquid chemical's propensity to burn. The only difference between a “flammable” and “combustible” liquid is the relative ease (temperature) with which the substance burns or supports burning. The assignment to combustible or flammable liquid categories is quite simple: if the flashpoint is between 37.8° C. and 93.3° C., it is a combustible liquid; if the flashpoint is below 38° C., it is a flammable liquid.        
The Canadian equivalent of OHSA concurs both in their assessment of the hazards of low flash point liquids and in the distinction between flammable and combustible.
The National Fire Protection Association in the U.S. has made their placement of fire hazard obviously the most important of the four classifications on their NFPA placards. Fire hazard is the red box and is located at the top of the four box square. They differentiate fire hazards on a scale of 0 to 4 where 0 means the chemical will not burn and 4 assigns a flash point below 23° C. This theme is repeated in the California Code of Regulations, Title 8, Section 5194 on Hazardous Substances and Processes.
In summary, it appears that many regulating bodies recognize the importance of higher flash point liquids and have defined the range of flash points, but the application of high flash point liquids to additives has not been available.
“Mechanisms and Techniques for the MgO Treatment of Coal-Fired Utility Boilers,” A. S. Dainoff and H. N. Schenck, presented at the Engineering Foundation Conference on Fouling and Slagging from Impurities in Combustion Gases, Copper Mt., Colo., Jul. 29-Aug. 3, 1984, provides a historical account of flue gas conditioning using magnesium in coal-fired plants.
WO/2007/053786 (J. E. Radway) discloses use of finely sized particles of alkaline earth carbonates or hydroxides in a water-based slurry. Small percentages (up to 5%) of oil solvents containing “overbased” organic-acid-neutralizing additives added to the slurry are disclosed. It is stated that:                Although they have been utilized in SO3 capture efforts, there have been no prior reports of their use for capturing either SO2 or toxic metals. Although emissions benefits can be obtained by the use of the so-called ‘overbased’ compounds, their much higher cost and combustibility make them a less attractive option for most applications. Additionally, the combustibility of the overbased materials requires hard piping as well as additional safety devices, each of which involves increased costs.        
Thus, there is a need in the art for a highly reactive oil dispersion having a high flash point for a coal or other fuels used in boilers.